The following correction was printed in the Observer's For the Record column, Sunday 4 October 2009

In the article below we said that "improved policing of the western Mediterranean, particularly the Canary Islands and southern Italy, has played a role". The Canary Islands are in the Atlantic Ocean.

Greek authorities are desperately trying to cope with a surge of migrants on to the country's islands which has left detention centres overflowing.

Last week, amid chaotic scenes, hundreds of migrants demonstrated against "inhuman conditions" in a detention camp on Mytilene, the capital of Lesbos, in a protest that saw hunger-striking minors setting fire to mattresses and attacking guards. The clashes highlighted the rising anger on island outposts that are being overwhelmed by a double influx of holidaymakers and illegal migrants.

According to senior immigration officials, Greece has now become the frontline of migration to the EU. "Greece is Fortress Europe's weakest link," said one EU official, who added that for traffickers bent on ferrying human cargo to the west, its borders were like a "big open door".

Last week in northern France, police used bulldozers to clear immigrants from the Calais camp known as the "jungle". But the problem there is dwarfed by the unfolding drama in the Greek islands.

Mytilene, off the coast of Turkey, and other tourist magnets can receive up to 500 "illegals" a day, according to authorities, and have become the favoured entry points into Europe for thousands from Afghanistan and Iraq.

"They're coming in by the boatload from Turkey at all hours of the night and day," said Nikoloas Zacharis, vice-prefect of Samos, another Aegean island. "It's uncontrollable."

Last week's "uprising," the latest in a series of revolts by immigrants, has provoked a huge row over Greece's treatment of "guests" it does not want.

"That children as young as 12 were on hunger strike in Greek detention is a gross indictment of the government's failure to care for them," said Simone Troller at Human Rights Watch.

Greece is not the only southern European country to be targeted by people smugglers. Spain, Italy and Malta have also been hit by an influx of immigrants but Greece and its islands are seen as Europe's easiest "backdoor" entrance. Last year an estimated 150,000 migrants, mostly from Asia but also from Africa, illegally entered Greece, police say.

Forced to cope with the country's porous land borders and 18,400 kilometres of unwieldy coastline, immigration officials are overstretched.

Tensions have been exacerbated by the extraordinary risks immigrants appear willing to take to cross the border. Those from war-torn Iraq and Afghanistan have frequently put their lives in danger to make the journey either in rickety rafts or on foot across minefields that still line Greece's northern land frontier with Turkey.

In recent years, an alarming number of pregnant women and parentless children have been among those crossing treacherous mountain passes and rough seas, according to human rights groups. Last year, as many as 3,000 minors – some of them as young as six and mostly from Afghanistan – were dumped by smugglers on remote Aegean isles.

In an attempt to staunch the human tide, Greek coastguard patrols have been equipped with high-speed boats and infrared tracking devices. France and Spain have dispatched helicopters to the area to help.

Acutely aware of the rising social tensions the influx has caused, the centre-right government, which faces an election on 4 October, has stepped up arrests with successive police sweeps in Athens' where rising crime has, increasingly, been blamed on migrants.

The arrests followed the announcement of draconian legislation in July, which included dramatically extending the amount of time undocumented migrants can be detained. And, despite widespread protests from Greeks and migrants groups over the prospect of "migrant concentration camps" being created, the conservatives have also floated the idea of detaining "illegals" in disused military facilities.

"The situation has reached crisis proportions, partly because detention centres are now so overcrowded," said Nikos Koplas, a lawyer who has long assisted refugees seeking asylum. "Locking them up is not the way forward. The answer lies with the EU. It's as if Greece is becoming a depot for illegal entries from all of Asia. It needs to share the burden."

In northern Europe capitals, where most illegal migrants head, the surge in arrivals has also caused growing consternation. Of 278 Afghan minors arrested last week in Calais, most entered Europe through Greece. Improved policing of the western Mediterranean, particularly the Canary Islands and southern Italy, has played a role.

"The main effect of more efficient patrols in the western Mediterranean is that we now have more people coming through the eastern Mediterranean," said Martin Baldwin-Edwards who runs the Mediterranean Migration Observatory at Athens' Panteion University.

"But most of the migrants are intent on moving on. When they see that conditions are not what they like or expect, they start heading deeper into Europe. Many prefer the UK because there's a whole mythology about it. They've heard from family and friends who are already there that it is a better democracy, with better conditions, plentiful jobs and fairer treatment of migrants."

Greece's notorious asylum process has the lowest acceptance rate in Europe. Of the 20,000 applicants last year, asylum was accorded to only 379.

Immigrants invariably complain that, with a backlog of more than 30,000 cases, they have no choice but to seek asylum elsewhere in Europe. As in France, authorities in Greece have tried to solve the problem by bulldozing makeshift camps, including one in the port city of Patras that, like the "jungle", was inhabited mostly by unaccompanied minors from Afghanistan.